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<br />Bosley Wash Drainage Master Plan Phase I: Watershed Floodplain Delineation
<br />_,= ~,m_"""":~""",~_,~""",-~.-",,,'T"'"'' "c",~,>",,, ",,,
<br />~ ~_~* .:nt:1~-;t~~~11";'_;:J:;:jj':<.>i0~"1;.t::::,:j;:A>;.:.';4"',':'~7':;'T':~.",:,..;,,:,-.'
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<br />The initial abstraction (I.) was assumed to be the SCS default. SCS curve numbers were
<br />
<br />
<br />developed using recommended values in TR-55 for various soil types and land uses, Percentages
<br />
<br />
<br />of impervious surfaces were assumed to be 2% in all lower basins and 0% in all upper basins
<br />
<br />since neither area had large areas of connected imperviousness, It was assumed that no baseflow
<br />
<br />was present in any of the channels including the irrigation ditches,
<br />
<br />4.3,1 Basin Development
<br />
<br />The project was intended to portray the floodplain produced by future hydrologic conditions,
<br />
<br />Although the lower reaches of the watershed downstream of 1-70 are comprised of sparse, rural
<br />
<br />land uses, it was assumed that the existing development will remain essentially unchanged,
<br />
<br />Significant growth is not anticipated, The land above 1-70 is privately owned and has a potential
<br />
<br />to develop, However, this area of the watershed was modeled with a very high curve number,
<br />
<br />primarily due to the very tight, poorly drained soils found here, Although future development
<br />
<br />
<br />will result in the presence of additional impervious areas from streets, roofs, and other artificial
<br />
<br />surfaces, it will not significantly affect runoff. The probable development is likely to include
<br />
<br />gardens and lawns that will mitigate the effects of the impervious surfaces,
<br />
<br />4.4 Diversion Simulation
<br />
<br />In many cases, the flow generated within the watershed significantly exceeds the conveyance
<br />
<br />capacity of the drainage system, In these cases, the flow often leaves the channel and flows
<br />
<br />downgradient toward another drainage channel. To simulate these conditions, the routing of the
<br />
<br />HMS model included diversions that routed flows in excess of the ditch capacity to the
<br />
<br />downstream junction where overflows eventually rejoin the channel. This enabled the
<br />
<br />representation of actual response to different storm events,
<br />
<br />4.5 Storage Simulation
<br />
<br />Hydrology calculations generated flows greater than the capacity of many of the culvert
<br />structures, For this reason, storage features were modeled in HMS that attenuated peak flow
<br />
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<br />Page 15
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